The present invention relates generally to optical transmission systems and, more particularly, to systems and methods for efficiently multiplexing/demultiplexing transmission channels in an optical transmission system.
Optical systems transmit information as optical signals through optical fiber. When sending optical signals over long distances, a number of optical channels may be simultaneously transmitted over a long length of fiber. Each of the optical channels correspond to a light source of a certain wavelength that is modulated with the data signal of the channel. The channels may be multiplexed together for transmission through the fiber.
FIG. 1 is a diagram illustrating an optical communication system 100. Transmitters 101-102 receive input information channels 120-121. Those skilled in the art will appreciate that many more than two, e.g., several hundred, channels may be used although only two are shown here to simplify the figure. Transmitters 101-102 may be long reach transmitters (LRTRs) that convert the input information channels 120-121 from electrical signals to optical information modulated around preset wavelengths. These optical channels are then combined by wavelength division multiplexer (WDM) 103 into a single WDM signal and transmitted over fiber link 115. Fiber connection 115 may include a number of optical fibers, each of which carries WDM signals, as well as repeaters 105 that, among other things, amplify the WDM signal.
The receiving side of communication system 100 includes WDM 110 and receivers 111-112. WDM 110 demultiplexes the received WDM signal into the original channels (wavelengths). Receivers 111-112 receive the demultiplexed optical channels and convert them back to electrical signals.
WDM signals traveling through fiber connection 115 experience chromatic dispersion. Dispersion refers to the fact that the different wavelengths in the WDM signal travel at different speeds in fiber connection 115. These different speeds cause the waveforms to become distorted as they travel through the fiber connection 115. In part, this dispersion can be managed by inserting fiber segments having appropriate dispersion characteristics along the fiber connection 115. While this reduces the average dispersion across the fiber connection 115, there remains some residual, wavelength dependent dispersion to be compensated.
One technique for compensating for this residual dispersion involves inserting a length of dispersion compensating optical fiber into the path of each optical signal. WDM 103 and/or WDM 110, for example, may include such a length of optical fiber for each of its input optical channels. An example of this technique can be illustrated by the situation in which each of a plurality of optical transmitters are connected to an array waveguide (AWG) through differing lengths of dispersion compensating fiber. Both the length and the type (i.e., positive or negative dispersion compensation) are selected based upon the expected residual dispersion associated with the wavelength (channel) at which each transmitter is operating. The required length of the dispersion compensating fiber can be relatively large (e.g., 80 km) for channels that require significant residual compensation. As more channels are added to the system, the amount of dispersion compensating fiber used in the WDM 103 and/or 110 quickly becomes a significant expense as well as increasing the size of the unit which causes it to use up valuable floor space in, e.g., a cable landing station. Moreover, the lossy nature of such parallel dispersion compensation schemes may require a large number of amplifiers.
Thus, there is a need in the art to be able to more efficiently multiplex and demultiplex optical channels in optical transmission systems.
Systems and methods consistent with the principles of the invention, among other things, provide for improved optical transmission/reception techniques.
One aspect consistent with the invention is directed to an optical transmission system for transmitting optical channels. The optical transmission system includes a first group of multiplexing units arranged in cascade with one another. The multiplexing units are configured to receive a predetermined number of the optical channels and output a WDM signal based on the predetermined number of received channels and based on a WDM signal from a previous one of the first group of multiplexing units in the cascade of multiplexing Units. Further, the optical transmission system includes dispersion compensation fibers each associated with one of the multiplexing units. The dispersion compensation fibers receive the WDM signals output from the associated multiplexing units. The dispersion compensation fibers have a length based on a length of fiber required to compensate for dispersion expected to be experienced by the input optical channels of the associated multiplexing unit and based on a length of fiber implemented in succeeding dispersion compensation fibers associated with the cascaded multiplexing units.
A second aspect consistent with the present invention is a method of transmitting optical channels through an optical fiber. The method includes combining the optical channels into a WDM signal via a group of multiplexing units implemented in cascade with one another. Additionally, the method includes compensating WDM signals output from each of the multiplexing units for optical dispersion. This compensation is performed by transmitting the optical channel output from a particular one of the multiplexing units through a predetermined length of dispersion compensating optical fiber, the predetermined length being set based on the length of optical fiber required to compensate for dispersion expected to be experienced by the WDM signal and based on a length of compensation fiber implemented in succeeding multiplexing units of the cascade of multiplexing units.
A third aspect consistent with the invention is directed to a system for receiving optical channels. The system includes demultiplexing units arranged in cascade with one another. Each of the demultiplexing units includes an input line configured to receive a first WDM signal that contains a number of optical channels, a number of output lines each configured to output a single optical channel, and a composite output line configured to output a second WDM signal. Further, the system includes dispersion compensation fibers associated with the demultiplexing units, each of the dispersion compensation fibers having a length based on a length of fiber required to compensate for dispersion experienced by the first WDM signal and based on a length of compensation fiber implemented in preceding dispersion compensation fibers associated with the cascaded demultiplexing units.
A fourth aspect consistent with the invention is directed to a system that includes modular multiplexing units, an underwater optical network, and modular demultiplexing units. The modular multiplexing units each include a first input line configured to receive a WDM signal, second input lines each configured to receive a single optical channel, and a composite output line configured to output a WDM signal containing information received at the first input line and the second input lines. The modular demultiplexing units include a third input line configured to receive a WDM signal, output lines each configured to output a single optical channel, and a second composite output line configured to output a WDM signal containing information relating to a group of optical channels.
A fifth aspect of the present invention is directed to a method for upgrading an optical communication system. A first set of WDM channels having a first average dispersion value associated therewith is initially provided to the optical communication system. Then, the optical communication system is upgraded by adding a second set of WDM channels having a second average dispersion value associated therewith. The first average dispersion value is less than said second average dispersion value.